fs/ext4: fix calling put_ext4 with truncated offset

Curently, we are using 32 bit multiplication to calculate the offset,
so the result will always be 32 bit.
This can silently cause file system corruption when performing a write
operation on partition larger than 4 GiB.

This patch address the issue by simply promoting the terms to 64 bit,
and let compilers decide how to do the multiplication efficiently.

Signed-off-by: Ma Haijun <mahaijuns@gmail.com>